In recent years, well-known cathode materials are lithium transition metal oxides and lithium transition metal phosphates for lithium ion batteries. Lithium transition metal phosphates have been of great interest as storage cathodes for rechargeable lithium batteries because of their high energy density, low raw material cost, environmental friendliness and safety. Among the lithium transition metal phosphates LiFePO4 possesses considerable importance because of its low cost, highest reversible capacity and excellent thermal stability. However, it suffers from low electrical conductivity and low lithium ion diffusion rate. Olivine type LiMnPO4 would also be of interest because of its high redox potential 4.05V Vs Li/Li+, which is compatible with present day lithium cobalt oxide material. However, LiMnPO4 is an insulator with 2 eV spin exchange band gap, which significantly lowers the electrochemical activity. Furthermore, this 4.05/4.1 V working potential is just below the limit of stability of the common organic electrolytes, which is used in lithium ion batteries thus allowing good cycle life without any degradation of the electrolyte in the battery.
Several groups have explored various methods to prepare electro-active LiMnPO4; there are very few reports which deliver performance greater than 100 mAhg−1. Few researchers have improved the performance of LiMnPO4 by the reduction of particle size using various synthesis methods. In this way U.S. patent No. 2008/0292522 A1 discloses the polyol process which yields 5 to 50 nm particles and provides excellent electrochemical characteristics. U.S. Patent No. 2009/0130560 A1 discloses the preparation of electrochemically active LiMnPO4 by sol-gel method. U.S. Patent No. 2009/0197174 A1 discloses the nano-sized crystalline LiMnPO4 powder with controlled morphology by direct precipitation at low temperature. T. Shiratsuchi et al [“Cathodic performance of LiMn1−xMxPO4 (M—Ti, Mg and Zr) annealed in an inert atmosphere”—T. Shiratsuchi, S. Okada, T. Doi, J. Yamaki, Electrochim. Acta 54 (2009) 3145”] and S. K. Martha et al [“LiMnPO4 as an advanced cathode material for rechargeable lithium batteries”—S. K. Martha, B. Markovsky, J. Grinblat, Y. Gofer, O. Haik, E. Zinigrad, D. Aurbach, T. Drezen, D. Wang, G. Deghenghi, I. Exnar, J. Electrochem. Soc. 156 (2009) A541”] have shown improved performance of LiMnPO4 by cation doping; Z. Bakenov et al [“Electrochemical Performance of nano-composite LiMnPO4/C cathode materials for lithium batteries”—Z. Bakenov, I. Taniguchi, Electrochem. Commun. 12 (2010) 75”] and S. K. Martha et al [LiMn0.8Fe0.2PO4: An advanced cathode material for rechargeablelithium batteries”—S. K. Martha, J. Grinblat, O. Haik, E. Zinigrad, T. Drezen, J. H. Miners, I. Exnar, A. Kay, B. Markovsky, D. Aurbach, Angew. Chem. Int. Edn. 48 (2009) 8559”] reported that electrochemically active carbon composites synthesized by using inert atmosphere. To summarize the recent reports, the nano-sizing process by means of sol-gel preparation and/or subsequent calcination with carbon in an inert atmosphere seems to be the most appropriate synthesis route to improve the performance of LiMnPO4.